In some content protection systems, an optical disc, such as a Blu-ray Disc, stores digital content (e.g., a movie) for playback on a host device (e.g., a Blu-ray Disc player). If the content is pirated, it is desired to be able to identify (“trace”) the host device that was responsible for generating the pirated copy. This process is referred to as “traitor tracing.” Once the traitor host device is identified, the host device's certificate and key can be revoked, so that the host device will no longer be able to decrypt (and possibly pirate) further content. Usual approaches of “traitor tracing” on optical disks rely on the selection of a variation of a sequence of data. As shown in FIG. 1, stored content (here, a movie) has a plurality of sequences of data (e.g., video frames), with each sequence of data having variation(s) (e.g., duplicate video frames that are nearly identical to one another but have some slight variation). Content variation can take the form of watermarked versions or digitally-edited versions that resist re-encoding.
There are many alternate navigation paths through these variations, and a particular host device is programmed to select a particular variation. That is, some variations will be selected and deciphered by certain host devices, while other variations will be selected and deciphered by other host devices. Accordingly, if a particular content title is pirated, the pirated version can be analyzed to identify what variations were selected. However, because of the limited storage space on a host device and the virtually-unlimited number of possible host devices, multiple host devices may output identical copies of the content, even though the content protection system is designed so that different host devices will not always get identical copies of different titles. Thus, a minimum number of pirated copies of a hacked title is needed to statistically identify where the breach happened.